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Murray floods shape as SA's worst disaster

South Australian authorities fear flooding of the Murray River could be the state's biggest natural disaster as assessments of the damage continue.

Emergency Services Minister Joe Szakacs said work to understand the impact of the flooding on homes, public buildings, roads and other infrastructure would take time.

"But it is entirely reasonable to say that this will be one of the most, if not the most, significant natural disaster in the state's history," the minister told reporters on Monday.

Initial forecasts suggested about 4000 properties would be flooded to some degree, including about 400 primary residences.

Work by the State Emergency Service over the next few days should provide more clarity on the number of homes impacted.

But Mr Szakacs said the damage extended to much more than homes.

"The damage from inundation is enormous and the task involved in assessing that is also very big," he said.

"Roads and infrastructure that is currently underwater needs to be assessed. That assessment will still take many weeks."

The flood peak has moved through the length of the Murray in SA, reaching the Lower Lakes late last week.

Water levels across the system have fallen, in some cases considerably, or are at least stable.

The bulk of the state's levee system has so far held up, including those protecting major assets such as the one at Renmark built near the local hospital and another at Mannum protecting businesses on the main street.

There have been 68 catastrophic levee failures and 168 major problems but most have been with structures protecting agricultural lands.

The flooding also closed about 120 roads across river communities with most of the state's ferry services also shut.

Of the 13 Murray ferries only those at Narrung, Cadell, Tailem Bend and Waikerie were still operating.

Also on Monday, the state government said the usual fees charged for emergency services to respond to false fire alarms would be waived across river communities.

Water inundating properties has resulted in some fire alarms being activated. In such circumstances, the property or business owner could be charged up to $645.

The government said the move was a gesture of goodwill from both the Metropolitan Fire Service and the Country Fire Service.

"Waiving the cost of fire alarm activations for flood-affected communities is just one of the many ways our emergency services are assisting the ongoing response," Mr Szakacs said.